The present invention relates to a magnetic recording apparatus in which recording and reproduction of information are performed magnetically and to a method of magnetic recording.
With improvement in computer processing speed in recent years, a magnetic recording apparatus (e.g., hard disk drive (HDD)) designed to record and reproduce information is required to improve recording speed and density more and more. However, there is a physical limitation in increasing the recording density.
In order to perform high-density recording with a magnetic recording apparatus, it is required to make small magnetic domains recorded in a recording layer. In order to distinguish the small recording magnetic domains, it is required that magnetic domain edges are smooth. In order to meet the above requirement, it is required to reduce the size of magnetic grains constituting the recording layer. It is also required to reduce the thickness of the recording layer for high-density recording, which also results in reducing the size of magnetic grains. However, when the magnetic grain size is reduced, magnetic anisotropy energy of the magnetic grain (i.e., the product of magnetic anisotropy energy density Ku and the volume of the magnetic grain) would likely to be smaller than thermal fluctuation energy. If the magnetic anisotropy energy of magnetic grains becomes smaller than the thermal fluctuation energy, magnetization of the recorded magnetic domains would be reversed again, thereby making it no more possible to retain the recorded information. This phenomenon is referred to as a thermal fluctuation limit or a superparamagnetic limit. It is conceivable, for the purpose of preventing the magnetization reversal due to the thermal fluctuation, to increase Ku of magnetic grains. However, when Ku of magnetic grains become higher, the coercive force of the magnetic grains would increase in proportion to Ku. Therefore, it may become impossible to reverse the magnetization by means of a magnetic field generated by an ordinary recording head.
An object of the present invention is to provide a magnetic recording apparatus and a method of magnetic recording, which make it possible to perform such high-density recording as exceeding the thermal fluctuation limit.
According to the present invention, there is provided a magnetic recording apparatus comprising a magnetic recording medium having a recording layer formed on a substrate, the recording layer being constituted by magnetic grains and a nonmagnetic material formed between them, a heating unit configured to heat the recording layer, and a magnetic recording unit configured to apply a magnetic field to the recording layer, wherein the magnetic recording medium, the heating unit and the magnetic recording unit are constituted so as to meet the following relationship:
T/RKu(T) less than 11200/(ln(t)+20.72)
where, setting that Ku(T) is magnetic anisotropy energy density of the recording layer at a temperature T, and Ku(Ta) is that at ambient temperature, RKu(T) represents a ratio Ku(T)/Ku(Ta), and t represents an elapsed time after the magnetic field application is completed.
In the magnetic recording apparatus of the present invention, the recording layer preferably has coercive force of 4 kOe or more at ambient temperature.
According to the present invention, there is provided a method of magnetic recording for a magnetic recording medium comprising a recording layer formed on a substrate and constituted by magnetic grains and a nonmagnetic material formed between them, the method comprising steps of heating the recording layer, and applying a magnetic field to the recording layer, thereby performing recording, wherein the steps meet the following relationship:
T/RKu(T) less than 11200/(ln(t)+20.72)
where, setting that Ku(T) is magnetic anisotropy energy density of the recording layer at a temperature T, and Ku(Ta) is that at ambient temperature, RKu(T) represents a ratio Ku(T)/Ku(Ta), and t represents an elapsed time after the magnetic field application is completed.
In the present invention, a method may be used, for example, in which the recording layer is heated such that RKu(Tmax) at the highest temperature Tmax becomes 0.01 or less in the heating step, and the recording operation is completed within 1 ns to 50 ns after the recording layer reaches the highest temperature in the recording step.
In the present invention, another method may be used in which the recording layer is heated such that RKu(T) becomes 0 before the recording layer reaches the highest temperature in the heating step, and the recording operation is completed within 20 ns to 100 ns after the recording layer reaches the highest temperature in the recording step.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.